1. Field of the Invention
The present invention relates to a four-wheel steering apparatus for vehicles and, more particularly, to a four-wheel steering apparatus for vehicles adapted to change a turning ratio of the rear wheels to the front wheels.
2. Description of the Prior Art
A four-wheel steering apparatus for a vehicle designed to turn the rear wheels in association with the front wheels in response to the operation of a steering wheel by an operator includes a front wheel turning mechanism and a rear wheel turning mechanism, and a steering angle of the steering wheel is arranged so as to be transmitted through the both turning mechanisms to the front wheels and the rear wheels as a turning angle. The steering angle of the steering wheel may be transmitted to the rear wheel turning mechanism by way of various means such as mechanical, electrical, fluid ones or a combination therewith.
In the four-wheel steering apparatus, a turning ratio of the rear wheels to the front wheels (that is, a ratio of a turning angle of the rear wheels to that of the front wheels) is not always constant and is commonly variable on the basis of a predetermined turning ratio characteristics. The turning ratio characteristics is set using a running state of the vehicle as a parameter. The parameters may include, for instance, a vehicle speed, the steering angle of a steering wheel, and a transverse G (an acceleration in the transverse direction) acting on the vehicle. U.S. Pat. No. 4,313,514 discloses the turning ratio characteristics set on the basis of vehicle speeds.
The turning ratio characteristics is not necessarily limited to one kind. A plurality of turning ratio characteristics may be set and one of them may be selected by a manual selection or the like. U.K. Pat. No. 2,152,452 discloses delay means for changing turning ratios gradually in selecting the turning ratio characteristics.
The direction in which the rear wheels are turned may be determined by a so-called same phase direction and a so-called reverse phase direction. The turning in the same phase direction is defined here, on the one hand, as the turning of the rear wheels in the direction identical to the direction in which the front wheels are turned. The turning of the rear wheels in the reverse phase direction is defined here, on the other hand, as the turning of the rear wheels in the direction opposite to the direction in which the front wheels are turned. Accordingly, in instances where a turning ratio is altered from one state to another, a changing direction being in the same phase direction, on the one hand, means that the direction in which the rear wheels are turned approaches to the direction in which the front wheels are turned. For example, where the front wheels are turned left, the rear wheels are also turned in the left-hand direction from the existing position. A changing direction being in the reverse phase direction, on the other, means that the direction in which the rear wheels are turned is parting from the direction in which the front wheels are turned. For instance, where the front wheels are turned left, the rear wheels are turned in the right-hand direction from the existing position. And a direction for changing the turning ratio has nothing to do with an existing state in which the rear wheels are turned, that is, the state in which the turning state is currently in the same phase or in the reverse phase. More specifically, assuming the existing state that the front wheels are turned left while the rear wheels are turned right, the state in which the rear wheels are turned left from the existing state is called an alteration of the turning ratio in the same phase direction and the state in which the rear wheels are turned farther right therefrom is called an alteration of the turning ratio in the reverse phase direction.
Extensive studies have been made to put the four-wheel steering apparatus into practice. As a result, it has been found that a rate of speeds at which the turning ratio is changed lies one of the points for commercialization.
From the point of view, we have already completed the invention as was previously disclosed and claimed in Japanese Patent Application No. 24,059/1986 filed Feb. 7, 1986 corresponding to U.S. patent application Ser. No. (not yet assigned) filed Feb. 4, 1987. This invention is briefly described to be designed so as to make a speed of changing a turning ratio slower when the rear wheel turning ratio is changed in the reverse phase direction than when the rear wheel turning ratio is changed in the same phase direction. More specifically, when the rear wheel turning ratio is changed in the same phase direction, on the one hand, the stability of a vehicle body is enhanced so that the turning ratio is designed to be changed quickly. When the changing direction is in the reverse phase direction, on the other hand, the stability of the vehicle body is redused so that the turning ratio is designed to be changed slowly.
It is, however, to be noted that, when there are seen influences which an alteration of the rear wheel turning ratio affects on the behaviour of a vehicle body, problems that a change in the behavior of a vehicle body offers are not restricted to a change in information that forces the alteration of the rear wheel turning ratios--say, elements such as a vehicle speed or a shift of characteristics. As a result of studies from this point of view, it has now been found that a sliding state of a road surface, i.e., a road surface friction coefficient .mu. affects a big influence on a change in the behavier of a vehicle body when the turning ratio is changed. It is thus to be noted that, even if the speeds of changing the turning ratios would be the same, the change in the behavior of the vehicle body becomes relatively larger when a road surface is more slidable or slippery, i.e., when the road surface friction coefficient .mu. is smaller, than when the road surface is less slidable or less slippery, i.e., when the road surface friction coefficient .mu. is larger.